FIG. 1 is cutaway view of a related art reactor pressure vessel 1, such as an ESBWR pressure vessel. Vessel 1 includes a core 15 within core shroud 10. Core shroud 10 separates upward flow of coolant through core 15 from downward flow in downcomer annulus 4, received from main feedwater line 3 and from liquid separated from steam separators and dryers. A feedwater cleanup system 12 may recirculate and filter feedwater from reactor vessel 1 into main feedwater line 3, and one or more control rod drives 50 may extend through a bottom of vessel 1 for control and monitoring of the conditions in core 15. As liquid coolant boils in core 15, a heated mixture of steam and water flows upward into steam separators and dryers that separate liquid water from the steam-water mixture rising therethrough. Liquid coolant from the steam separators and dryers is directed into downcomer annulus 4 between the outer wall 10 and inner wall of vessel 1 for recirculation. The dried steam exiting steam dryers is then directed into main steam lines 2 for electrical power production.
As described in ESBWR Design Control Document, Tier 2, 2013, incorporated by reference herein in its entirety, flow through core 15 is derived from a heat balance calculation; there is no direct measurement of fluid flow through core 15. The balance of heat and overall energy created and extracted from core 15 allows operators to estimate or model whole core flow. For example, for an ESBWR at 100% power, 31,553 tons/hr of coolant may enter a lower core section. This derived core flow may be directly presented to operators as a measurement of plant status as well as a diagnostic aid in transient conditions.